The present invention relates to ceria powders and particularly to ceria powders of the type used to fine and polish optical and semiconductor surfaces.
It is well known that in order to produce a satisfactory optical surface, it is necessary that the surface be free of scratches and have as low an R.sub.a as possible. This R.sub.a measurement is the average distance between the highest and lowest points on the surface perpendicular to the plane of the glass sheet being polished. Thus, accepting that the surface will not be totally flat at the submicron scale, it is a measure of the variation between highest and lowest points. Clearly the lower the figure the better for optical clarity and freedom from distortion.
In polishing processes a slurry of abrasive particles in a liquid medium, (usually water-based), is placed in contact with the surface to be polished and a pad is caused to move across the surface in predetermined patterns so as to cause the abrasive in the slurry to polish the surface. The present invention relates to ceria particles useful in such processes.
Various slurry formulations have been proposed in the art. U.S. Pat. No. 4,576,612 produces its slurry in situ in controlled amounts by providing a pad with a surface layer comprising the abrasive particles in a resin which gradually dissolves during use to liberate the polishing particles. The particles declared to be useful include cerium oxide ("ceria"), zirconium oxide ("zirconia") and iron oxide.
EP 608 730-A1 describes an abrasive slurry for polishing a surface in an optical element which comprises an abrasive selected from alumina, glass, diamond dust, carborundum, tungsten carbide, silicon carbide or boron nitride with particle sizes up to one micron.
U.S. Pat. No. 5,693,239 describes an aqueous slurry for polishing and planarizing a metallic workpiece which comprises submicron particles of alpha alumina together with other softer form of alumina or amorphous silica.
U.S. Pat. No. 5,804,513 teaches a planarization slurry comprising ceria and alumina or silica, where the particle sizes are 2 microns or less and the ceria component is from 5 to 40% of the total weight.
U.S. Pat. No. 4,106,915 teaches a method of polishing glass using abrasive particles dispersed in a cured, unsaturated polyester resin wherein the abrasive can be ceria.
U.S. Pat. No. 4,601,755 teaches ceria-based glass polishing compositions incorporating a rare earth pyrosilicate.
A considerable amount of art also exists in the related field of slurry formulations for chemical mechanical planarization, ("CMP"), or polishing of semiconductor substrates and again, these commonly employ the same abrasives with variations in components of the dispersion medium.
The use of ceria in such applications is therefore well known and widely practiced. The ceria is commercially obtainable in reasonably small particle sizes of a little over a micron but there is always a drive to ever more perfect surfaces and this propels a move to finer ceria particle sizes still. There is however an associated problem with this move in that the ceria tends to be sensitive to agglomeration and such agglomerates can cause scratching of the surface being polished. Thus commercially available ceria tends to a reach a limiting value of around 10 A for surface roughness, (R.sub.a), when used in a slurry form to polish glass surfaces.
The present invention is based on the discovery of a form of ceria with unique characteristics that leads it to be highly successful in resisting agglomeration and polishing to R.sub.a levels of 5 .ANG. or even lower and a method by which such ceria can be obtained.